STUDIES ON THE INTERACTION BETWEEN NUCLEIC-ACIDS AND CATIONIC PORPHYRINS .1. EFFECT OF THE BASICITY OF THE PORPHYRIN ON THE BINDING MODE TO THE NUCLEIC-ACID

Interaction between nucleic acids (Calf Thymus, poly(dG-dC).poly (dG-dC), poly(dA-dT).poly(dA-dT)) and two cationic porphyrins (tetrakis-[4-{(3-trimethylaminopropyl)-oxy} phenyl]-porphyrin, TTHETAOPP, and tetrakis-[4N-{3-trimethyl-aminopropyl} pyridyl]porphyrin, TTHETA(py)P) has been studied by UV/Vis and CD spectroscopy as well as viscosimetry. The octapositively charged TTHETA(py)P was found to be an intercalator with the GC base-pairs though its size is much bigger than that of the classical intercalator TM(py)P(4). On the contrary, TTHETAOPP having nearly the same size as TTHETA(py)P, is not able to intercalate even at low salt concentration: it is an outside binder with both GC and AT base-pairs and shows a great tendency to aggregate. Both binding modes were supported by the appropriate changes in optical spectral and by the viscosimetric data. The results clearly show the important role of the basicity on the binding mode of porphyrin to DNA: the low pK(a) (approximately 1 for TTHETA(py)P) prefers intercalation whereas high pK(a) (4.6 for TTHETAOPP) makes the porphyrin an outside binder.